U.S. Pat. Nos. 4,640,352 and 4,886,118 disclose conductive heating of subterranean formations of low permeability that contain oil to recover oil therefrom. Low permeability formations include diatomites, lipid coals, tar sands, and oil shales. Formations of low permeability are not amiable to enhanced oil recovery methods such as steam, carbon dioxide, or fire flooding. Flooding materials tend to penetrate formations that have low permeabilities preferentially through fractures. The injected materials bypass most of the formation hydrocarbons. In contrast, conductive heating does not require fluid transport into the formation. Oil within the formation is therefore not bypassed as in a flooding process. When the temperature of a formation is increased by conductive heating, vertical temperature profiles will tend to be relatively uniform because formations generally have relatively uniform thermal conductivities and specific heats. Transportation of hydrocarbons in a thermal conduction process is by pressure drive, vaporization, and thermal expansion of oil and water trapped within the pores of the formation rock. Hydrocarbons migrate through small fractures created by thermal stress and by the expansion and vaporization of the oil and water.
U.S. Pat. Nos. 3,113,623 and 3,181,613 disclose gas fired heat injection burners for heating subterranean formations. These burners utilize porous materials to hold a flame and thereby spreading the flame out over an extended length. Radiant heat transfer from a flame to the casing is avoided by providing the porous medium to hold the flame. But for combustion to take place in the porous medium, the fuel gas and the combustion air must be premixed. If the premixed fuel gas and combustion air were at a temperature above the autoignition temperature of the mixture, they would react upon being premixed instead of within the porous medium. The formations utilized as examples of these inventions are only up to fifty feet thick and below only about fifteen feet of overburden. The fuel gas and the combustion air are therefore relatively cool when they reach the burner. The burner would not function as it was intended if the formation being heated were significantly deeper.
U.S. Pat. No. 5,255,742 discloses a flameless combustor useful for heating subterranean formations that utilizes preheated fuel gas and/or combustion air wherein the fuel gas is combined with the combustion air in increments that are sufficiently small that flames are avoided. Creation of NO.sub.x is almost eliminated, and cost of the heaters can be significantly reduced because of less expensive materials of construction. Preheating the fuel gas according to the invention of patent '742 results in coke formation unless CO.sub.2, H.sub.2, steam, or some other coke suppressant is added to the fuel gas. Further, start-up of the heater of patent '742 is a time consuming process because it must operate at temperatures above the autoignition temperature of the fuel gas mixture. Start-up requires long periods of very low flow-rate operation before temperatures would be sufficiently high for normal operation.
Catalytic combustors are also known. For example, U.S. Pat. No. 3,928,961 discloses a catalytically-supported thermal combustion apparatus wherein formation of NO.sub.x is eliminated by combustion at temperatures above auto-ignition temperatures of the fuel, but less than those temperatures at which result in substantial formation of oxides of nitrogen.
Metal surfaces coated with oxidation catalyst are disclosed in, for example, U.S. Pat. Nos. 5,355,668 and 4,065,917. These patents suggest catalytic coated surfaces on components of a gas turbine engine. Patent '917 suggests use of catalytic coated surfaces for start-up of the turbine, and suggests a mass transfer control limited phase in the start-up operation.
It is therefore an object of the present invention to provide a combustion method and apparatus which is flameless, and can be easily ignited and brought up to operating temperatures. In another aspect of the present invention, it is an object to provide a combustion method and apparatus wherein formation of NO.sub.x is minimal. It is another object of the present invention to provide a method which has a high level of thermal efficiency.